Energy Costs of Pollution Control

Combining oxygen and air preheat may sound risky, but may be a way to higher efficiencies if carefully monitored by modern controls, and provided NOx generation in not increased. 

Early days of pollution control aimed principally at smoke abatement, that is, particulate emission control. For installations using solid fuels, it was often necessary to change to more expensive gaseous or liquid fuels, which later were less expensive. As better designs evolved to reduce particulates, users benefited because more complete combustion was achieved. 

The formation of NO (which later becomes NO2, both of which are collectively known as NOx) is a chemical process with a reaction rate that is a function of temperature. The NO formation rate doubles for every 16°F (9°C) of reaction temperature rise if sufficient nitrogen and oxygen ions are available. Therefore, prime goals of combustion engineers should be to (a) reduce reaction (flame) temperature as much as possible and b) use mixing configurations that minimize concurrent availability of N and O. 

Water or stream spraying are considered only emergency measures. External fgr is more effective than in-furnace recirculation of combustion chamber gases because its gases are usually much cooler, but it actually has to have a higher cost than most people realize, as shown in the following example 5.3 and its summary tabulation. 

Example 5.3 (Cost of fgr) A furnace burning natural gas has 1800 F (1255 C) flue gas exit temperature with 10% excess air. Use %available heat calculations to compare fuel costs for Cases a toe discussed next. 

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ENERGY COSTS OF POLLUTION CONTROL 235 TABLE 5.6. Heat contents of gases . Courtesy of North American Mfg. Co. 

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